Human Serum Albumin ("HSA") is a major component of blood plasma and can be used as a plasma or serum substitute to treat burns, haemorrhagic shock and other conditions. Methods have been described whereby rHA can be produced by fermentation of microorganisms, i.e. suitable strains of bacteria and yeasts, into which the gene for HSA has been introduced by recombinant DNA technology (GB- A-2147903).
It is recognized by those skilled in the art that the expression vectors utilized to transform the microorganisms should be so constructed as to cause the microorganism to secrete rHA into the surrounding culture fluid. This is desirable because intracellular rHA, in common with many other heterologous proteins accumulated by microorganisms, is produced in an inactive, insoluble form from which the native protein can be obtained only with great difficulty (see, for example, M. Latta et al 1987 Bio Technology 5 1309-1314).
It is also recognized by those of ordinary skill in the art that, in order to maximize the productivity of the recombinant manufacturing process, it is desirable to grow the microorganisms in a highly agitated aerated fermenter thereby achieving high concentrations of cells and product. Unfortunately, these conditions leave the secreted protein exposed to physical, chemical and enzymatic degradation in the extracellular medium such that the quantity of recombinant protein recovered from the fermenter is much less than might have been expected from the performance of the microorganisms in less intensive conditions. This phenomenon is characteristic of the production of rHA.
Therefore, there exists a need to maximize the amount of rHA that is produced under such intensive conditions, particularly where such maximizing does not adversely affect the overall efficiency of the process. This need has been fulfilled in accordance with the present invention.